Two sets of contoured, flexible ribs allow the upper back to flex more than the lower, for exceptional comfort and support. Adjustable height 18 3/32" - 22 1/2". One-piece molded polypropylene shell with a pneumatic cylinder. Chair swivels 360°. Pedestal base has a five-prong construction. Five 2" diameter, dual-wheel, hooded swivel casters. Optional 5-pack of fixed glides and 5-pack of soft-wheel casters are available.